Most DSLRs already have a wide selection of lenses available, including those made by the camera maker and also from independent lens makers like Sigma, Tamron and Tokina. The third-party lenses include lower-cost alternatives to some of the camera manufacturers' options, along with lenses that aren't available in the camera makers' lines, and support most or all camera features, including autofocusing and automatic exposure control. But adapters provide access to even more lenses, specifically those not available in mounts for the camera in question.
The recent rise of the mirrorless, interchangeable-lens camera has made lens adapters even more popular—partly because some of the mirrorless camera makers don't offer a wide range of lenses for these cameras yet (nor do the independent lens makers) and partly because it allows use of higher-performance lenses not currently offered for their mirrorless systems.
Mirrorless camera makers are aware of the benefits an adapter can bring to the party. Sony offers adapters that permit use of lenses for the company's DSLR cameras (as well as legacy Konica Minolta Maxxum lenses) on their NEX mirrorless cameras, greatly expanding the lens lineup. The recently introduced LA-EA2 adapter not only permits using these DSLR lenses, but it also incorporates a quick phase-detection AF system similar to the one in Sony's SLT-A65 translucent-mirror DSLR.
Olympus and Panasonic offer adapters that let you mount standard Four Thirds System lenses on their Micro Four Thirds System mirrorless cameras, retaining the cameras' contrast-based autofocusing with many of them. And Samsung offers a K-mount adapter that lets you use the wide range of Pentax K-mount lenses with its mirrorless NX cameras.
FLANGE BACK DISTANCEThis odd-sounding term refers to the distance between the lens mount and the image plane in the camera (the image sensor in a digital camera). Each camera manufacturer designs its DSLRs or mirrorless cameras with a specific flange back distance. If you wish to use a lens made for another camera on your camera, you have to consider this distance.
If the flange back distance of your camera is smaller than the distance for which the lens was designed, that's not too bad. The appropriate adapter generally will make up the difference, and if it doesn't, the worst that can happen is the lens will be able to focus "beyond infinity."
Problems arise when the flange back distance for the lens and adapter is greater than that of the camera. In that case, the lens won't focus all the way to infinity. You can use it for closer work, but not for landscapes and other distant subjects. Some adapters include an optical element to permit focusing to infinity, but generally this also reduces image quality.
The mirrorless, interchangeable-lens cameras have very short flange back distances because they don't have the bulky mirror box found in DSLRs. So these cameras can be used with any lens for which an adapter can be found—including lenses from old film SLRs.